Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426 (Reexamination Certificate 907) taught that the speed-crossover relationship of radiographic elements containing imaging units coated on opposite sides of a film support (hereinafter also referred to as dual coated radiographic elements) can be improved by employing one or more spectrally sensitized high tabularity silver halide emulsions. High tabularity silver halide emulsions are those in which the tabular grains exhibit a mean tabularity (T) of greater than 25, T being defined by the relationship: EQU T=D/t.sup.2 (R 1)
where
D is the effective circular diameter (ECD) in micrometers of the tabular grains and PA1 t is the thickness in micrometers of the tabular grains.
When spectrally sensitized tabular grain emulsions are compared to nontabular grain emulsions in a dual coated radiographic element format, spectrally sensitized tabular grain emulsions produce reduced crossover as compared to nontabular grain emulsions of matched sensitivity (speed) and increased speed as compared to nontabular grain emulsions exhibiting matched grain surface area. Based on this speed-crossover relationship advantage as well as a number of other advantages, including improved speed-granularity relationships, increased silver image covering power both on an absolute basis and as a function of binder hardening (allowing simplification of processing), more rapid developability, and increased thermal stability, tabular grain emulsions in general and high tabularity emulsions in particular have found wide acceptance.
Notwithstanding the numerous advantages of dual coated radiographic elements containing spectrally sensitized tabular grain emulsions, a disadvantage has arisen in attempting to employ tabular grain emulsions having mean tabular grain thicknesses of less than 0.2 micrometer (hereinafter also referred to as thin tabular grain emulsions) in that staining of the fully processed radiographic elements can occur, attributable to failure to remove the spectral sensitizing dye or dyes adequately during processing. The reason for increased dye stain is that the surface area of thin tabular grains is quite high in relation to their volume. On the other hand, to be effective as a sensitizer the ratio of a dye to grain surface area must be at least 30 percent of monomolecular coverage, where "monomolecular coverage" indicates the amount of dye required to provide a layer one molecule thick over the entire surface area of the silver halide grains present in an emulsion. In a number of instances the thicknesses of tabular grains selected for tabular grain emulsions have been increased, with consequent performance degradation attributable to the consequent reduction in grain tabularity, so that the grain surface area per silver mole in the coatings is reduced and the amount of spectral sensitizing dye can be reduced to achieve tolerable stain levels while retaining high levels of spectrally sensitized speed. This balancing fails to achieve the full advantages that would otherwise be available for thin, high tabularity tabular grain emulsions.
As dual coated radiographic elements are most commonly employed, each element is mounted between a pair of intensifying screens for exposure. An imagewise pattern of X-radiation striking the screens causes them to emit longer wavelength radiation that is primarily responsible for producing the developable latent image in the dual coated radiographic element. Since the ability of silver halide to absorb X-radiation directly is limited, the presence of the screens greatly increases the imaging speed of the system and as a result greatly reduces patient exposure to X-radiation during diagnostic imaging.
Among the most efficient and widely used of phosphors for constructing intensifying screens are terbium activated gadolinium oxysulfide phosphors. These phosphors emit principally in the 540 to 555 nm region, exhibiting a peak emission at 545 nm. To capture efficiently the light emitted by these phosphors when incorporated in intensifying screens it is necessary to choose one or a combination of spectral sensitizing dyes for incorporation in the imaging emulsion layers that exhibit peak light absorption in the same spectral region in which the phosphors exhibit peak emission.
Spectral sensitizing dyes are adsorbed to silver halide grain surfaces to permit the grains to form a developable latent image when exposed to electromagnetic radiation in a spectral region to which the silver halide grains lack native sensitivity. Spectral sensitizing dyes are almost universally chosen from among polymethine dyes and are most typically cyanine or merocyanine dyes. Benzimidazolocarbocyanine dyes are very efficient at utilizing light energy and their high basicity allows them to be protonated and removed in processes which use acidic solutions, leaving low residual stain. These dyes function best as J-aggregates on the silver halide grain surface. Such benzimidazolocarbocyanine aggregates, however, generally absorb light at 560 to 590 nm, the long green region of the spectrum. As such, it has been heretofore necessary to use a different class of dyes, e.g. the oxacarbocyanines or benzimidazolooxacarbocyanines, for sensitization in the mid-green region. These dyes, however, being less basic tend to leave unacceptably high levels of retained dye after processing. Another disadvantageous feature of many benzimidazolo-carbocyanines is their relatively low oxidation potential, which can lead to poor storage stability of the radiographic elements in which they are incorporated. This poor keeping is observed as an increase in fog and/or a loss of photographic speed with storage or incubation of the photographic material.
Known benzimidazolocarbocyanine, oxacarbocyanine, and benzimidazolooxacarbocyanine dyes are illustrated by Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426 (Reexamination Certificate 907); Ukai et al U.S. Pat. No. 4,510,235; and Ikeda et al U.S. Pat. No. 4,837,140.